The present invention relates generally to gas turbine engines, and, more specifically, to noise attenuation in fans thereof
In a turbofan gas turbine engine configured for powering an aircraft in flight, air is pressurized in a fan and discharged for producing propulsion thrust. A portion of the fan air is channeled into a core engine including a multistage axial compressor that further pressurizes the air which is then mixed with fuel and ignited in a combustor that discharges hot combustion gases downstream in turn to high and low pressure turbines. The high pressure turbine powers the compressor, and the low pressure turbine powers the fan.
The fan rotor blades are substantially larger in outer diameter than the compressor rotor blades for moving a large volume of air to create propulsion thrust. The row of fan blades is surrounded by a fan nacelle which is typically supported to the core engine by a row of supporting radial struts. And, disposed downstream from the fan blades is a row of stator vanes inside the fan nacelle for deswirling the fan air prior to discharge through the fan nozzle or outlet. In an alternate configuration, the fan vanes and struts may be integrated in common vane-strut members.
A primary source of fan noise for aircraft engines is the interaction of the wakes shed from the fan blades with the fan vanes disposed downstream therefrom. The row of fan blades rotates relative to the stationary fan vanes with a blade passing frequency in which each of the blade wakes produces a corresponding non-uniformity in velocity at the row of fan vanes. The rotor wake-vane interaction produces unsteady loading on the stator vanes, which in turn produces pressure waves that propagate upstream and downstream as sound radiating at the blade passing frequency and its harmonics.
Furthermore, turbulence in the fan wakes also interacts with the downstream stator vanes generating unsteady loading and corresponding sound waves at a wide spectrum of frequencies related to the energy spectrum of the wake turbulence itself.
Accordingly, the aerodynamic interaction between the fan blades and vanes creates substantial fan noise during operation which must be limited to comply with various governmental noise regulations. Fan noise may be typically reduced by increasing fan diameter and reducing fan tip speed and pressure ratio. Fan noise may also be decreased by increasing the axial spacing between the fan blades and vanes. And, fan noise may also be reduced by preferentially selecting the relative number of blades and vanes in each row to provide cut-off of the blade passing frequency tones. The fan nacelle also typically includes noise absorbing acoustic liners for additional noise attenuation.
However, these noise attenuation techniques affect engine performance, affect engine size and weight, and vary in effectiveness.
Accordingly, it is desired to provide an improved turbofan for noise attenuation.
A turbofan engine includes a row of fan blades disposed upstream from a row of stator vanes, and is powered by a core engine. The fan blades are serrated for mixing wakes therefrom to attenuate fan noise.